1. Field of the Invention
The present invention relates to a spun-like fasciated yarn having good heat durability maintaining excellent mechanical strength and less creep deformation even under a high temperature condition exceeding 300.degree. C.
2. Description of the Prior Art
It is well-known that wholly aromatic polyamide fibers such as poly-para-phenyleneterephthalamide or poly-meta-phenyleneisophthalamide which is on the market under the trade name "Kevlar.RTM." from Du Pont de Nemours and Co. of USA or "Cornex.RTM." from Teijin Limited of Japan, has superior mechanical strength and high modulus as well as good heat durability and anti-erosive properties. Due to the above-mentioned advantages, they are widely utilized in the industrial field.
These fibers are used not only in filament form but also preferably in the form of spun yarn. For example, a bag filter is one of the suitable usages of the wholly aromatic polyamide fibers because the bag filter has to often be exposed to a high temperature and erosive atmosphere. In this case, a fabric woven from the spun yarn of such a fiber is expected to be more advantageous than a filament yarn fabric because of a better filtration ability caused by a relatively looser structure and a larger thickness of the spun yarn than of the filament yarn. However, according to a conventional spinning process, a resultant spun yarn cannot satisfactorily be applied to the aforesaid object due to the reasons explained below.
Since polymers of the above-mentioned heat durable fibers do not show a clear melting point but have a wide range decomposition point close to the former, it is very difficult to adopt a melt spinning system to prepare a fiber. Accordingly, the fiber can be formed only by the dry spinning or wet spinning of a solution in which the polymer is dissolved by a suitable solvent. Under such conditions, it is more advantageous to have a thicker tow for staple fibers than to have a plurality of thinner filament yarns. Since the tow for staple fibers has a very large thickness of, for example, from several dozen thousand denier to several hundred thousand denier, a higher draw ratio is hardly attainable. This insufficient drawing as well as a residual solvent in the fiber unremoved during the spinning process result in an undesirable lower tensile strength and a lower Young's modulus of the resultant fiber than expected from a structure of a recurring unit of the polymer.
Further, to have a spun yarn, the tows thus obtained are cut into staple fibers after being crimped and, thereafter, are subjected to a conventional complicated spinning process comprising scutching, carding, drawing, doubling, twisting, etc. As a result, the fibers in the spun yarn are considerably disoriented and deformed. This causes, along with the crimps thereof, a greater elongation and lower strength of the yarn. Especially, since the wholly aromatic polyamide fiber lacks a spinnability due to its hardness and rigidity, the abovesaid tendency is accelerated. Accordingly, the spun yarn has an undesirable creep deformation, especially in a high temperature atmosphere, compared to a filament yarn. This is the reason why the conventional spun yarn of the wholly aromatic polyamide fibers is unsatisfactory in the above-mentioned object.